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Comment by supermatt | Hacker News Reader

I had someone come and measure boundaries for some land on Friday, supposedly with centimetre precision. Does this mean that the markers they installed are likely in the wrong place?

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It depends... were there any markers already present? In a past life I worked as a surveyor, and if a marker was present(it looks like a silver-dollar on the ground, with a spike holding it in place, these can last 100+ years), we would use that to measure from. If, however, they set up what looked like a tripod with a oval-shaped dish on top, that's a GPS receiver, and could be affected by this outage.

Link: here are some survey markers for sale, linking it so you can see a picture of what they commonly look like: https://www.berntsen.com/Surveying/Survey-Markers/Aluminum-S...

No - they placed new markers (orange disks on long spikes) driven flush into the ground, but they were walking around using GPS to do this (with supposedly centimetre precision, which I understand is what GNSS is for). This is in Eastern EU.

No, those markers work by listening to the GPS signals for an extended period of time and maybe PPK or perhaps an alternative network that's not satellite based.

So it would be regular GPS assisted with some basestation at a known location to boost accuracy, as opposed to GNSS (Galileo)?

Yes, I'm not in land surveying myself, but my company builds georeferenced 3D models of buildings at high resolution and sub cm precision, and we never rely solely on GPS information. There's too many things that can go wrong with GPS to reliably fly a drone around a building for example.

"GNSS" just means satellite positioning systems in general, and e.g. "GPS" and "Galileo" are examples of GNSS systems.

At least regular receivers (e.g. in phones) utilize signals from multiple satellite constellations simultaneously, not sure if this is the case here as well.

Yes, exactly. They use the signal as received at a nearby marker previously surveyed with high accuracy to calculate a local 'correction' to the GPS/GNSS signal, and give that correction to the roving units.

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